The Automobile Association is worried agricultural vehicle operators who work long hours will put motorists at risk.

"They might be driving home after 18 hours on a tractor," principal adviser Mark Stockdale, of Wellington, said.

"What confidence can motorists have that an agricultural machinery operator working long hours on a farm is not fatigued when he's on a public road?

"The agricultural industry says it's difficult to work within the regulations [restricting hours of work], but the trade-off is public safety."

Inglewood contractor Billy Moratti said operators would still be restricted to 13 hours a day with regular half-hour breaks unless they gained an exemption, perhaps after a long period of wet weather. Contractors would hire more staff and equipment so health and safety guidelines were observed, even though that increased costs. Operators would be collected from farms and delivered home if necessary.

"If we're travelling at 40kmh, it means we take longer to get there and hold up more traffic."

Early-morning traffic between Stratford and New Plymouth was heavy, and agricultural vehicles could not be driven continuously inside the white line on the left of the road.

"So you slow down and move over to let the traffic pass, do another kilometre and let another few pass."

Mr Stockdale said the AA preferred the existing 30kmh speed limit and opposed a 50kmh limit.

Agricultural vehicles represent 1.15 per cent of New Zealand's vehicle fleet and were involved in 0.2 per cent of accidents between 1997 and 2010. There were 28 deaths, 73 seriously injured people and 175 minor injuries.

The NZ Transport Agency is seeking public feedback till the end of the month on the changes.

Agricultural vehicles exceeding 40kmh will need an annual WOF and must be driven by the holder of an agricultural endorsement or class 2 licence.